Method and apparatus for scheduling packets in an orthogonal frequency division multiple access (OFDMA) system

ABSTRACT

A method and apparatus for scheduling packets in an orthogonal frequency division multiple access (OFDMA) system is provided. More particularly, the presently described embodiments relate to a scheduling system whereby packets are prioritized based on packet delay information. The system also handles poor geometry users in an improved manner.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for scheduling packetsin an orthogonal frequency division multiple access (OFDMA) system. Moreparticularly, the presently described embodiments relate to a schedulingsystem whereby packets are prioritized based on packet delayinformation.

While the invention is particularly directed to the art of schedulingpackets in an OFDMA environment, and will be thus described withspecific reference thereto, it will be appreciated that the inventionmay have usefulness in other fields and applications. For example, theinvention may be used in other environments where prioritizing packetsis advantageous.

By way of background, OFDMA systems are utilized as an air interface infourth generation (4G) systems for wireless communication. Whereflexibility in providing bandwidth for multiple users is desired, OFDMAprovides a solution. OFDMA requires that users be assigned to certainbandwidth designations, regardless of the needs of the users at aparticular moment. So, the flexibility resides in the ability to servemore users at a given time. Scheduling the users on the system becomesimportant to manage the system effectively.

However, OFDMA systems have the capability to be used to transmitvoice-over-internet-protocol (VOIP) calls. To minimize control signalingoverhead, including signaling messages in OFDMA systems, an approachcalled persistent assignment is supported in Ultra Mobile Broadband(UMB) standard for VOIP applications. Persistent assignment isparticularly useful where the number of simultaneously scheduled usersin a sector is very high.

One implementation of persistent assignment scheduling is that, once avoice-over IP user begins a talk spurt, and is allocated with a resourceunit, the user maintains the same resource unit until the end of thetalk spurt. Also, the transmission format remains the same for all ofthe data transmitted until a persistent assignment expires.

To explain, with reference to FIG. 1, a frequency spectrum 10 is shownin an OFDMA system. As shown, the frequency spectrum includes pilotsub-carriers 12 as well as user data sub-carriers 14 and 16. Forcompleteness, the guard bands 18 are also shown. It should beappreciated that the data sub-carriers or interlaces, are used totransmit data in the system. If persistent assignment is used, thesesub-carriers are consistently used for a user during a, for example, atalk spurt.

A disadvantage of prior art of implementing voice-over IP in OFDMAsystems is that the system does not take into account of packet delaywhen scheduling a new user. As a result, quality of service may sufferbecause higher priority calls may have to wait while the resources areused in a fixed manner in the OFDMA system.

As such, it is desirable to have a system whereby packets can bescheduled on a more efficient basis.

The present invention contemplates a new and improved that resolves theabove-referenced difficulties and others.

SUMMARY OF THE INVENTION

A method and apparatus for scheduling packets that are known OFDMAsystem are provided.

In one aspect of the invention the method comprises determining if apacket is to be retransmitted, retransmitting the packet if it is to beretransmitted, if the packet is not to be retransmitted, determining ifthe packet is subject to a persistent assignment, if the packet issubject to persistent assignment, transmitting the packet usingpersistently assigned resources in a persistently assigned format and,if the packet is not subject to persistent assignment, prioritizingtransmission of the packet based on packet delay information.

In another aspect of the invention, the prioritizing comprisesscheduling a predetermined number of non-persistent users on a prioritylist.

In another aspect of the invention, the method further comprisesdetermining whether a packet corresponds to a poor geometry user, and,segmenting the packet if the packet belongs to a poor geometry user.

In another aspect of the invention, retransmitting comprisesretransmitting through a physical layer.

In another aspect of the invention, the method further comprisesdetermining whether a user should be added to a persistent assignmentlist.

In another aspect of the invention, the method further comprisesdetermining whether a maximum number of users have been assigned apriority based on delay.

In another aspect of the invention, the packet delay parameters arebased on delay experienced by user for which packets are not beingretransmitted or persistently assigned.

In another aspect of the invention, the method comprises determiningwhether a packet corresponds to a poor geometry user, segmenting thepacket into smaller packets if the packet belongs to the poor geometryuser, and, transmitting the segmented smaller packets.

In another aspect of the invention, the method further comprisesprioritizing transmission of the packet based on packet delayinformation.

In another aspect of the invention, the method comprises determiningwhether a packet corresponds to a poor geometry user, segmenting thepacket into smaller packets if the packet belongs to a poor geometryuser, scheduling each segmented packet for transmission independently ifthe packet is segmented, determining if the packet is to beretransmitted, retransmitting the packet if it is to be retransmitted,if the packet is not to be retransmitted, determining if the packet issubject to a persistent assignment, if the packet is subject topersistent assignment, transmitting the packet using persistentlyassigned resources in a persistently assigned format, and, if the packetis not subject to persistent assignment, prioritizing transmission ofthe packet based on packet delay information.

In another aspect of the invention, the transmitting of the smallersegmented packets comprises prioritizing the transmission based onpacket delay information.

In another aspect of the invention, retransmitting comprisesretransmitting through a physical layer.

In another aspect of the invention, the method further comprisesdetermining whether a user should be added to a persistent assignmentlist.

In another aspect of the invention, the method further comprisesdetermining whether a maximum number of users have been assigned apriority based on delay.

In another aspect of the invention, the packet delay parameters arebased on delay experienced by users for which packets are not beingretransmitted or persistently assigned.

In another aspect of the invention, a means is provided to implement themethods described herein.

Further scope of the applicability of the present invention will becomeapparent from the detailed description provided below. It should beunderstood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art.

DESCRIPTION OF THE DRAWINGS

The present invention exists in the construction, arrangement, andcombination of the various parts of the device, and steps of the method,whereby the objects contemplated are attained as hereinafter more fullyset forth, specifically pointed out in the claims, and illustrated inthe accompanying drawings in which:

FIG. 1 illustrates a frequency spectrum for an OFDMA system.

FIG. 2 is a flow chart illustrating the presently described embodiments.

FIG. 3 is a flow chart illustrating the presently described embodiments.

FIG. 4 is a block diagram illustrating a system into which the presentlydescribed embodiments may be incorporated.

DETAILED DESCRIPTION

The presently described embodiments, in one form, are directed to ascheduling system for packets in an OFDMA environment. The systemprioritizes transmission of packets based on delay information. Thedelay information relates to users and is readily available in thesystem. The system also handles poor geometry users in an improvedmanner.

More particularly, in one form, the system is operative to determine ifa packet or packets for a user are to be retransmitted. If so, thepacket(s) is retransmitted. If the packet(s) is not to be retransmittedbut is subject to persistent assignment, the packet(s) is processedusing persistently assigned resources in a persistently assigned format.In a case where the subject packet(s) is not to be retransmitted and isnot subject to persistent assignment, the transmission of the packet(s)is prioritized based on packet delay information that is availablewithin the system.

To further enhance the scheduling techniques of the presently describedembodiments, a determination is made as to whether a particular packetcorresponds to a poor geometry user. A poor geometry user is a user thatis typically on the edge of a coverage area for a wireless network.Packets for such users are typically retransmitted more than four timesand the packet delay would build up if the transmission format is notreduced. However, with the presently described embodiments, once apacket is determined to be a packet for a poor geometry user, thetransmission format can be reduced via packet segmentation into smallerpackets to keep the packet delay within certain bound.

Referring now to the drawings wherein the showings are for purposes ofillustrating the exemplary embodiments only and not for purposes oflimiting the claimed subject matter, FIG. 2 is a flow chart illustratingthe presently described embodiments. It should be appreciated, however,that the methods according to the presently described embodiments may beimplemented in a variety of suitable manners in a variety of suitableenvironments. For example, the methods of FIGS. 2 and 3 may beimplemented using any of a number of acceptable software routines and/orhardware configurations. The resources that are used may be at leastpartially centralized or distributed throughout the network in anadvantageous manner. In one form, the routines and appurtenant hardwareaccording to the presently described embodiments may be implemented in abase station of the network, or may be implemented in a gateway-typedevice within the network or may be implemented in a Radio NetworkController (RNC) or similar device. Still further, the primaryfunctionality of the system may be housed in the primary switchingelement of the network.

With reference to FIG. 4, a communications system 100 is illustrated, inaccordance with one embodiment of the present invention. This representsonly one example of a system into which the present application may beincorporated—the configuration of the system may well vary from oneimplementation to the next. For illustrative purposes, thecommunications system 100 of FIG. 4 is a wireless telephone system thatemploys a broadband data standard commonly known as cdma 2000Evolution—Data Optimized (EV-DO), although it should be understood thatthe present invention may be applicable to other systems that supportdata and/or voice communication. The communications system 100 allowsone or more mobile devices 120 to communicate with a data network 125,such as the Internet, and/or a public telephone system (PSTN) 160through one or more base stations 130 and additional circuitry and/ornetwork elements 138, such as a Radio Network Controller (RNC). The basestations 130 may take a variety of forms and may include a number ofcomponents and functionality not shown for convenience. For example, aphysical layer is not specifically shown (but is shown in FIG. 2).However, in one form, as representatively shown for convenience, atleast one of the base stations includes counters 140 and 142, a database144, a scheduling module 146 and various queues 148. Both (or all) basestations in the network may take this form. It is to be understood thatthese element are shown to illustrate overall functionality and may takedifferent forms and may even be disposed within different elements ofthe system. It should further be understood that the database may havestored therein a variety of information including lists (such aspersistent assignment lists, retransmission lists and non-persistentassignment lists). These lists may be prioritized based on selectedcriteria (such as delay) and, of course, may be finite in length,allowing for a maximum number of users to be listed thereon. The queues148 may be included in the database as well, but, at least in one form,maintain information on packet delay. The mobile device 120 may take theform of any of a variety of devices, including cellular phones, personaldigital assistants (PDAs), laptop computers, digital pagers, wirelesscards, and any other device capable of accessing the data network 125and/or the PSTN 160 through the base station 130. In this environment,the presently described embodiments, including those described inconnection with FIGS. 2 and 3 (or combinations thereof), may beimplemented in a variety of manners. For example, as noted above, thetechniques described herein may be implemented within the base stations130. The scheduling module 146 may be provided therein to providescheduling functions and to control and conduct the techniques describedherein by, among other things, accessing the other noted components ofbase stations 130.

Referring back now to FIG. 2, a flow chart illustrating a method 200according to the presently described embodiments is shown. The method200 is initiated by resetting each channel or interlace (at 202) at thebeginning of a talk burst of a user (by, e.g., the scheduling module146). This, of course, may be accomplished using any of a variety ofsuitable techniques. A counter “i” (e.g., counter 140 of FIG. 4) isinitially set to zero (at 204). This counter is used to identify users 1through N. It is then determined whether the packet or packets for theuser are to be retransmitted (at 206). As is well known in the field, apacket may be re-transmitted for a variety of reasons. These reasonsinclude, but are not limited to, situations where a “NO ACKOWLEDGE” or“NACK” message is received on behalf of a particular user. If the packetis to be retransmitted, the user is added to a re-transmission list (at208), and the packet is retransmitted using the physical layer (at 210).The re-transmission list may be maintained in any known manner at aconvenient location in the system including, for example, in thedatabase 144. Also, the process of transmitting in this environment maybe accomplished in any suitable manner through the physical layer.

If the packet or packets are not to be retransmitted, a determination ismade as to whether a persistent assignment is to be used (at 212). Ifso, a persistent assignment is implemented to ultimately transmit thepacket through the physical layer (at 214). As noted above, thetechnique of persistent assignment may be implemented by the OFDMAsystem to allow the same set of resources to be dedicated to aparticular user for an amount of time to accommodate, for example, atalk burst of the user. By maintaining such resources and the messageformat, the amount of signaling is reduced.

The user is then added to a persistent assignment list (at 216). Thepersistent assignment list may be maintained in any known manner at aconvenient location in the system including, for example, in thedatabase 144. The counter “i” is incremented (at 218) once theprocessing for the user is complete. Recall that this counter identifiesusers. Then, it is determined whether “i” is less than “N”, the totalnumber of users in the sector, for example (at 220). If users stillrequire processing, then steps 206 through 220 are repeated. If no usersremain, the scheduling of persistently assigned users is completed andthe packets are transmitted through the physical layer.

If the packet or packets are not to be retransmitted and not subject topersistent assignment, the user of the packet(s) is added to thenon-persistent list (at 222). The list may be prioritized based on delayand have a finite number of entries. The non-persistent list may bemaintained in any known manner at a convenient location in the systemincluding, for example, in the database 144. The packet or packets isthen sorted based on packet delay among a fixed group of users (at 224).Packets experiencing longer delays are given higher priority fortransmission. Packet delay information may be maintained in the systemin a variety of different manners. In one form, a packet queue (such asqueue 148) maintains delay information for particular users. In anotherform, a database (e.g., database 144) in the system is updated withpacket delay information. A counter “j” (e.g., counter 142) is set tozero (at 226). This counter keeps track of the number of non-persistentassignment messages in one interlace. In one form, the maximum number ofnon-persistent assignment messages for one interlace is “M.” Aninterlace-tile pair is then assigned to transmit the packet (at 228) andthe counter “j” is incremented (at 230). A determination is then made asto whether “j” is less than “M” (at 232). If so, steps 228-232 arerepeated. If not, the scheduling is completed and, ultimately, physicallayer transmission is completed (at 210).

In other forms of the presently described embodiments, a determinationmay be made before step 202 that a poor geometry user (described in moredetail below) is involved in a transmission. That being the case,additional steps may be taken to ensure the desired cell coverage whilemaintaining the packet delay within certain bound.

In this regard, with reference to FIG. 3, a method 300 is illustrated.The method 300 is initiated by identifying a poor geometry user (at 302)(by, for example, element 138 of FIG. 4). Such identifying may beaccomplished using a variety of suitable techniques, as a function ofthe network design. The techniques may include monitoring variousparameters, such as signal strength, position, number ofretransmissions, . . . etc. A poor geometry user is typically a userthat is on the edge of a cell, or pico-cell, area or a user that issufficiently blocked from the cell tower to make transmission to and/orfrom that user difficult. In these types of poor geometry circumstances,multiple retransmissions are typically experienced to achieve certaincell coverage. Such excessive number of retransmissions would causeunbounded packet delay buildup, which significantly impacts systemperformance. While extended frames have been previously used to solvethe coverage problem, typical implementation requires complicatedresource management across, for example, three time-contiguousinterlaces, which imposes constraints on scheduling for data-VoIP mixedscenarios.

So, if a poor geometry user is identified, the packets for that user aresegmented into smaller packets (at 304). The segmentation may take avariety of forms. In one form, however, a full-rate voice packetsegmentation is carried out so users can take more time resources tomeet packet delay requirements. Besides the lower transmission formatthat can be used to achieve certain cell coverage, each segmented packetcan be treated independently in physical layer, thus allowing moreefficient resource usage by the system. For example, with smallerpackets, it is easier for the system to find suitable channels orinterlaces upon which the packet can be transmitted. The segmentedpackets are then scheduled for transmission by, for example, thescheduling module 146 and transmitted (at 306).

It should be appreciated that the method described in connection withFIG. 3 is well suited for the method described in connection with FIG. 2in a variety of manners. Both of them fit well with the dynamic resourceallocation nature for data (including VoIP) packets in OFDMA systems,especially when a user enters talk spurt and is to be newly scheduled.In one form, the steps of identifying a poor geometry user (e.g. step302) during the beginning of a talk spurt and segmenting the packets, ifnecessary (e.g. step 304), could be performed prior to the step ofresetting each interlace (e.g. step 202). The smaller, segmented packetscould then be prioritized, as in step 224, based on delay informationbefore transmission. The physical layer transmission (at 210) could alsobe used instead of step 306.

It will be understood that the presently described embodiments providemany advantages. For example, more flexibility in frequency allocationmay be experienced. Also, a simpler scheduling design can be achieved.Also, increased cell coverage may result from implementation of thepresently described embodiments.

The above description merely provides a disclosure of particularembodiments of the invention and is not intended for the purposes oflimiting the same thereto. As such, the invention is not limited to onlythe above-described embodiments. Rather, it is recognized that oneskilled in the art could conceive alternative embodiments that fallwithin the scope of the invention.

1. A method for scheduling packets in an orthogonal frequency divisionmultiple access (OFDMA) system, the method comprising: determining if apacket is to be retransmitted; retransmitting the packet if it is to beretransmitted; if the packet is not to be retransmitted, determining ifthe packet is subject to a persistent assignment; if the packet issubject to persistent assignment, transmitting the packet usingpersistently assigned resources in a persistently assigned format; and,if the packet is not subject to persistent assignment, prioritizingtransmission of the packet based on packet delay information.
 2. Themethod as set forth in claim 1 wherein the prioritizing comprisesscheduling a predetermined number of non-persistent users on a prioritylist.
 3. The method as set forth in claim 1 further comprising:determining whether a packet corresponds to a poor geometry user; and,segmenting the packet if the packet belongs to a poor geometry user. 4.The method as set forth in claim 1 wherein retransmitting comprisesretransmitting through a physical layer.
 5. The method as set forth inclaim 1 further comprising determining whether a user should be added toa persistent assignment list.
 6. The method as set forth in claim 1further comprising determining whether a maximum number of users havebeen assigned a priority based on delay.
 7. The method as set forth inclaim 1 wherein the packet delay parameters are based on delayexperienced by user for which packets are not being retransmitted orpersistently assigned.
 8. A method for scheduling packets in anorthogonal frequency division multiple access (OFDMA) system, the methodcomprising: determining whether a packet corresponds to a poor geometryuser; segmenting the packet into smaller packets if the packet belongsto the poor geometry user; and, transmitting the segmented smallerpackets.
 9. The method as set forth in claim 8 further comprisingprioritizing transmission of the packet based on packet delayinformation.
 10. A method for scheduling packets in an orthogonalfrequency division multiple access (OFDMA) system, the methodcomprising: determining whether a packet corresponds to a poor geometryuser; segmenting the packet into smaller packets if the packet belongsto a poor geometry user; scheduling each segmented packet fortransmission independently; if the packet is not segmented, determiningif the packet is to be retransmitted; retransmitting the packet if it isto be retransmitted; if the packet is not to be retransmitted,determining if the packet is subject to a persistent assignment; if thepacket is subject to persistent assignment, transmitting the packetusing persistently assigned resources in a persistently assigned format;and, if the packet is not subject to persistent assignment, prioritizingtransmission of the packet based on packet delay information.
 11. Themethod as set forth in claim 10 wherein the transmitting of the smallersegmented packets comprises prioritizing the transmission based onpacket delay information.
 12. The method as set forth in claim 10wherein retransmitting comprises retransmitting through a physicallayer.
 13. The method as set forth in claim 10 further comprisingdetermining whether a user should be added to a persistent assignmentlist.
 14. The method as set forth in claim 10 further comprisingdetermining whether a maximum number of packets have been assigned apriority based on delay.
 15. The method as set forth in claim 10 whereinthe packet delay parameters are based on delay experienced by users forwhich packets are not being retransmitted or persistently assigned. 16.A system for scheduling packets in an orthogonal frequency divisionmultiple access (OFDMA) system, the system comprising: means fordetermining if a packet is to be retransmitted; means for retransmittingthe packet if it is to be retransmitted; means for if the packet is notto be retransmitted, determining if the packet is subject to apersistent assignment; means for if the packet is subject to persistentassignment, transmitting the packet using persistently assignedresources in a persistently assigned format; and, means for if thepacket is not subject to persistent assignment, prioritizingtransmission of the packet based on packet delay information.
 17. Asystem for scheduling packets in an orthogonal frequency divisionmultiple access (OFDMA) system, the system comprising: means fordetermining whether a packet corresponds to a poor geometry user; meansfor segmenting the packet into smaller packets if the packet belongs tothe poor geometry user; means for scheduling each segmented packet fortransmission independently based on packet delay information; and, meansfor transmitting the segmented smaller packets.